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Sites associated with Anonymous have also come under attack

Key websites of the Tunisian government have been taken offline by a group that recently attacked sites and services perceived to be anti-Wikileaks.

Sites belonging to the Ministry of Industry and the Tunisian Stock Exchange were amongst seven targeted by the Anonymous group since Monday.

Other sites have been defaced for what the group calls "an outrageous level of censorship" in the country.

The group also recently targeted the websites of the Zimbabwean government.

What is Anonymous?

'Anonymous' describes itself as an 'internet gathering'. The term is used to describe a leaderless collective of people who come together online, commonly to stage a protest.

The groups vary in size and make-up depending on the cause. Members often identify themselves in web videos by wearing the Guy Fawkes masks popularised by the book and film V for Vendetta.

Its protests often take the form of disrupting websites and services.

Its use of the term Anonymous comes from a series of websites frequented by members, such as the anarchic image board 4Chan.

These allow users to post without having to register or provide a name. As a result, their comments are tagged "Anonymous".

In the past, groups have staged high-profile protests against plans by the Australian government to filter the internet and the Church of Scientology.

The latter spilled over into the real world with protests by masked members outside churches. An offshoot of Anonymous called Project Chanology focuses purely on this cause.

Many Anonymous protests tackle issues of free speech and preserving the openness of the net.

Those attacks were reportedly in retaliation after the president's wife Grace Mugabe sued a Zimbabwean newspaper for $15m (£9.6m) over its reporting of a cable released by Wikileaks that claimed she had made "tremendous profits" from the country's diamond mines.

The attacks, which started in the run up to the New Year, hit the government's online portal and the official site of Robert Mugabe's Zanu-PF party.

"We are targeting Mugabe and his regime in the Zanu-PF who have outlawed the free press and threaten to sue anyone publishing Wikileaks," the group said at the time.

In an open letter published online, Anonymous said that it had launched distributed denial of service (DDoS) attacks to highlight a spate of recent riots that have taken place over youth unemployment in the country as well as net and press censorship.

The country has reportedly stepped up its control of the web in light of the recent violence.

The retaliatory DDoS attacks used to knock the government's websites offline do so by bombarding them with so much data that they can no longer respond to legitimate page requests.

Security researcher Graham Cluley said the group, which encourages members to download a piece of software to launch the attacks, had selected its targets in discussions in an Internet Relay Chat (IRC) forum on Sunday.

But he warned people against being tempted to take part.

"Anyone considering signing-up to join in the attacks on the websites of various governments would be wise to remember that participating in a DDoS attack is against the law," he said.

What is a DDoS attack?

A Distributed Denial of Service (DDoS) attack aims to make websites inaccessible

Attackers commonly use networks of compromised computers - called a botnet - that they control to launch the attacks

Anonymous, however, encourage members to install a piece of software on their own computers to launch attacks

By overwhelming the target site with requests, the attackers can ensure that genuine visitors cannot reach the site

These requests look like genuine web traffic so can be hard to filter out

Typically, such attacks have been aimed at high-profile websites, such as those belonging to government departments, banks and political organisations

They are illegal in the UK

As well as the DDoS attacks, Anonymous said it had taken other measures.

"We have accessed one of their websites and defaced it by placing our Open Letter to the Government of Tunisia on the main page

"In addition, we have taken steps to ensure that Tunisians can connect anonymously to the internet and access."

The Tunisian government has not responded to a request for comment on the attacks.

In a twist, websites associated with Anonymous are also under DDoS attack, according to Netcraft.

The firm said that it had seen attacks against the AnonNews.org site, and the anarchic message board 4Chan, commonly frequented by members of Anonymous.